The answer could be:
Disembark.
My infix spells an alphabet,
And can put things to action
This could be Em, the letter 'M', and the prefix for putting things to action. (eg. Empower)
My prefix means to criticise,
And undo or inaction
This could be Dis, as in 'diss', to criticise, as well as the prefix to remove or revert.
My postfix is a sound
And to a plant, it is bound
This could be Bark, as in tree bark, and the sound.
As a verb I am found
I place you back on the ground
Hence, Disembark, as in disembarking from a vehicle, or to be put back on the ground.